Over the past decade, there has been a concerted effort to reduce atmospheric pollution caused by volatile solvents which are emitted during the painting process. However, it is often difficult to achieve high quality, smooth coating finishes, such as are required in the automotive industry, without using organic solvents, which contribute greatly to flow and leveling of a coating. In addition to achieving near-flawless appearance, automotive coatings must be durable and chip resistant, yet economical and easy to apply.
Currently, in the automotive industry the coating system which provides a good balance between economy, appearance and physical properties is a system having four individual coating layers. The first coating is a corrosion resistant primer which is applied by electrodeposition and cured. The next coating is a primer/surfacer which is spray applied and then cured. The third coating is a spray-applied colored basecoat. The basecoat is generally not cured before the application of the final coating, the clear coat which is designed to provide toughness and high gloss to the system. The process of applying one layer of a coating before the previous layer is cured is referred to as a wet-on-wet ("WOW") application.
U.S. Pat. No. 5,262,464 discloses a primer which can be dried at ambient conditions for 60 minutes and coated with a waterborne basecoat and two component, low VOC clearcoat (column 7, line 60 to column 8, line 44). The primer coating composition includes an aqueous dispersion of a thermoplastic anionic polyacrylate or polyurethane. The polyacrylate has functional carboxylic acid or anhydride groups which are neutralized with ammonia. The polyurethane is also neutralized with ammonia or an amine to be dispersible in water.
It is desirable, however, to use a thermosettable primer/surfacer coating to provide better adhesion to the substrate. Unfortunately, conventional thermosettable waterborne primer/surfacer compositions need to be cured before the basecoat is applied, increasing cost by requiring major capital investment in ovens and large amounts of energy.
The automotive industry would derive a significant economic advantage from an inexpensive coating process which provides a coated composite having good adhesion, chip resistance and smoothness, yet which can be applied wet-on-wet-on-wet ("WOWOW"), i.e., a process in which the primer/surfacer is not heated or is heated only for a short time at a low temperature to evaporate some of the water and/or solvent remaining in the primer/surfacer after it has been applied without significant crosslinking thereof.